1. Field of Invention
The present invention relates to a permanent magnet, more particularly an Nd-Fe-B sintered magnet, as well as to a method for producing the same.
In the Nd-Fe-B magnets there are melt-quenched magnets and sintered magnets. Essentially, the melt-quenched magnet is magnetically isotropic. There is a proposed method for rendering the melt-quenched magnet anisotropic, residing in crushing a strip obtained by melt-quenching to produce powder, hot-pressing and then die-upsetting the powder. This method is however not yet industrially carried out, since the production steps are complicated.
2. Description of Related Arts
Nd-Fe-B sintered magnet is developed by the present inventor et al. It has outstanding characteristics in that it exhibits excellent magnetic property in terms of 50 MGOe of maximum energy product (BH)max in a laboratory scale and 40 MGOe even in a mass production scale; and, the cost of raw materials is remarkably cheaper than the rare-earth cobalt magnet, since the main components are such cheap elements as Fe and B, and Nd (neodymium) and Pr (praseodymium), whose yielding amount is relatively high in the rare earth elements. Representative patents of the Nd-Fe-B sintered magnet are Japanese Unexamined Patent Publication No. 59-89401, Japanese Unexamined Patent Publication No. 59-46008 (Japanese Examined Patent Publication No. 61-34242, Japanese Patent No. 14316170, Japanese Unexamined Patent Publication No. 59-217003), U.S. Pat. No. 4,597,938 and European Patent No. EP-A-0101552. As an academic paper, there is "New Material for permanent magnets on a base of Nd and Fe (invited)", M. Sagawa et al, J. Appl. Phys., 55, No. 6, Part II, p 2083/2087 (March, 1984).
A permanent magnet is exposed, after magnetization, to an inverse magnetic field due to various reasons. A permanent magnet must have a high coercive force in order that irreversal demagnetization does not occur even after exposure to a strong reverse magnetic field. Recently, along with size reduction of and efficiency-increase of appliances, inverse magnetic field applied to the appliances is increasing more and more. In a motor, for example, a magnet is exposed after its magnetization to a strong self demagnetization, until it is mounted in a yoke. After mounting, the magnet is exposed, during energization, to an inverse magnetic field from a coil and to a magnetic field which corresponds to the permeance of a magnetic circuit. The inverse magnetic field from the coil reaches the maximum at start. When a motor stops due to an excessive load and is then immediately restarted by switching on, the most severe load is applied to the magnet. In order to withstand this and suppress the irreversible demagnetization field, a permanent magnet must have a coercive force as high as possible.
Under recent progress of appliances, the level of load, which is required for magnets, is unseen heretofore. In an appliance for extracting a strong emission light in an accelerator referred to as an undulator or wiggler, there is a proposal of structure that completely magnetized plates of permanent magnets are bonded with one another in such a manner that N poles face one another and alternately S poles face one another. Obviously, for such application the permanent magnets having a high coercive force are necessary. There is a trend that such use of permanent magnets is increasing more and more in future.
The coercive force also has a relationship with the stability of a permanent magnet. When a permanent magnet is allowed to stand after magnetization, irreversible demagnetization occurs little by little. In order to lessen the irreversible change of magnetization with time, coercive force should be as higher as possible than the inverse magnetic field under using state. Accordingly, there are more and more requests for permanent magnets having a high coercive force.
In addition, when a permanent magnet is exposed under high temperature, since the coercive force lowers at a high temperature, its temperature characteristics become important. Temperature coefficient of coercive force, which exerts an influence upon the temperature-characteristics of coercive force, is from 0.3 to 0.4%/.degree. C. for the melt-quenched strip magnet, and is slightly lower than this value for the melt-quenched and then anisotropically treated strip magnet. Temperature coefficient of coercive force is 0.5%/.degree. C. or more for the sintered magnet.
The temperature-coefficient of a sintered magnet varies depending upon a measurement temperature range and is greater at a lower temperature. The temperature coefficient (.beta.) of the coercive force herein is determined by the following formula. ##EQU1## .DELTA.iHc: difference (kOe) in the intrinsic coercive force (iHc) in the temperature change of from 20.degree. C. to 120.degree. C.
iHc: intrinsic coercive force at 20.degree. C. (kOe) PA1 .DELTA.T: temperature difference (100.degree. C.).
The measuring interval of temperature coefficient of coercive force (iHc) is set from 20.degree. to 120.degree. C., since the temperature interval becomes 100.degree. C.
Since the temperature coefficient of coercive force (iHc) is 0.5%/.degree. C. and is very high for the Nd-Fe-B sintered magnet, the intrinsic coercive force (iHc), hereinafter referred to as the coercive force (iHc), is lowered at a high temperature to make the magnet unusable. Specifically speaking, in the case for permeance coefficient =1, the limiting usable temperature of the Nd-Fe-B sintered magnet is approximately 80.degree. C. The Nd-Fe-B sintered magnet, whose temperature coefficient of coercive force (iHc) is 0.5%/.degree. C. or more and is very high irrespective of the composition, could therefore not be used at a high temperature and as parts of automobiles and motors used at temperature raising to 120.degree.-130.degree. C. during use.
Various devices have been made to enhance the coercive force of Nd-Fe-B sintered magnet. Coercive force (iHc) of the Nd-Fe-B sintered magnet having standard composition Nd.sub.15 Fe.sub.77 B.sub.8 is approximately 6 kOe. Considering that the residual magnetization (Br) of this magnet exceeds 12 kG, the coercive force (iHc)=6 kOe is too low so that its application scope is extremely limited. One of the most successful methods for enhancing the coercive force was heat treating the Nd.sub.15 Fe.sub.77 B.sub.8 sintered magnet, subsequent to sintering, at 600.degree. C., which increased the coercive force (iHc) to 12 kOe (M. Sagawa et al. J. Appl. Phys. vol. 55, No. 6,15, March 1984). This was a great achievement but higher coercive force is necessary from a practical point of view.
Japanese Unexamined Patent Publication No. 61-295355 discloses a Nd-Fe-B sintered magnet containing a boride phase of BN, ZrB.sub.2, CrB, MoB.sub.2, TaB.sub.2, NbB.sub.2, and the like. According to the explanation in this publication: it is effective for providing a high coercive force to lessen the grain size of a sintered body as possible; the boride particles added to the main raw materials suppress of grain growth during sintering; and, the coercive force (iHc) increases by 1-2 kOe due to the suppressed grain growth. In addition, according to the above publication, it is indispensable for obtaining a permanent magnet having improved magnetic properties that the R.sub.2 Fe.sub.14 B phases be surrounded along their boundary by R rich phases and B rich phases.
Japanese Unexamined Patent Publication No. 62-23960 discloses to suppress the grain growth by using such borides as TiB.sub.2, BN, ZrB.sub.2, HfB.sub.2, VB.sub.2, NbB, NbB.sub.2, TaB, TaB.sub.2, CrB.sub.2, MoB, MoB.sub.2, Mo.sub.2 B, WB, WB.sub.2, and the like. Nevertheless, only slight enhancement of coercive force is attained by the technique of suppressing the grain-growth due to addition of these borides. Such borides incur generation of Nd.sub.2 Fe.sub.17 phase which is magnetically detrimental. The addition amount of borides is therefore limited to a relatively small amount. Most of the borides, such as BN and TiB, impede the sintering and densification of the sintered product.
Explorations have also been made for methods of enhancing the coercive force by means of additive element(s). Virtually all of the elements in Periodic Table have been tested. The most successful method among them was the addition of heavy rare-earth elements, such as Dy. For example, when 10% of Nd of Nd.sub.15 Fe.sub.77 B.sub.8 is replaced to provide Nd.sub.13.5 Dy.sub.1.5 Fe.sub.77 B.sub.8, the coercive force (iHc) amounts to .gtoreq.17 kOe. Because of the discovery that Dy is effective for enhancing the coercive force (iHc), Nd-Fe-B sintered magnet is at present being used in a broad field of application.
Various additive elements other than the heavy rare-earth elements were also tested. For example, in Japanese Unexamined Patent Publications Nos. 59-218704 and 59-217305, V, Nb, Ta, Mo, W, Cr and Co were added and heat treatment was devised in various ways. However, the coercive force (iHc) obtained is low and the effects obtained were exceedingly inferior to those attained by Dy. Al is effective for enhancing the coercive force (iHc), although not as prominent as Dy and Pr, but disadvantageously drastically lowers Curie point.
Although Dy provides excellent coercive-force characteristics, the abundance of Dy in ores is approximately 1/20 times of Sm and is very small. If Nd-Fe-B sintered magnets with Dy additive are mass-produced, Dy is used in amount greater than the amounts of respective elements balanced in the rare-earth resources. There is a danger that the balance is destroyed and the supplying amount of Dy soon becomes tight.
Tb and Ho, which belong to rare-earth elements as Dy, have the same effects as Dy, but, Tb is even more rare than Dy and is used for many applications such as opto-magnetic recording material. The effects of Ho for enhancing the coercive force (iHc) is exceedingly smaller than that of Dy. In addition, the resource of Ho is poorer than Dy. Tb and Ho therefore practically speaking cannot be used.
As is described hereinabove there are two methods for producing Nd-Fe-B series magnet. According to the melt-quenching method, alloy melt is blown through a nozzle and impinged upon a roll rotating at a high speed to melt-quench the same. A high coercive force is obtained by this method by means of adjusting the rotation number of a roll and the conditions of post-heat treatment after the melt-quenching.
The melt-quenched magnet has a grain size of 0.1 .mu.m or less and is fine. Therefore, even if a melt-quenched magnet has the same composition as the Nd-Fe-B sintered magnet, the former magnet is characterized by a higher coercive force than the latter magnet. In addition, mechanism of coercive force of the melt-quenched magnet is pinning type and hence is different from the nucleation type of sintered magnet. The temperature coefficient of coercive force (iHc) of melt-quenched magnet is 0.3-0.4%/.degree. C. and is hence lower than 0.5%/.degree. C. or more of the sintered magnet. This is also a feature of the melt-quenched magnet. Contrary to this, the melt-quenched magnet involves a problem in the properties other than the coercive force. That is, the melt-quenched magnet is isotropic in the state as it is. Special technique is necessary for rendering the melt-quenched magnet to anisotropic. The isotropic magnet exhibits Br approximately 1/2 times and (BH).sub.max approximately 1/4 times those of anisotropic magnet and cannot provide high performance. The hot-pressing and then die upsetting method causes a deformation work which aligns the crystal orientation. Although a high performance is obtained by this method, the process is complicated.
Generally, the production method of sintered magnet is for example as follows.
(a) Melting
An alloy ingot having a target composition or alloy ingots having a few kinds of the compositions are obtained.
(b) Rough Crushing
Roughly crushed powder under 35-100 mesh is obtained by a jaw crusher and a disc mill or the like.
(c) Fine pulverizing
Fine powder having an average grain size of 3 .mu.m or less is obtained by a jet mill or the like.
(d) Press under magnetic field
Compressing is carried out for example in a magnetic field of 13 kOe with a pressure of 2 ton/cm.sup.2.
(e) Sintering
Sintering is carried out in vacuum or Ar gas at 1000.degree. to 1160.degree. C. for 1-5 hours.
(f) Heat treatment
Heat treatment is carried out at 600.degree. C. for 1 hour.
Nd-Fe-B sintered magnets produced by such methods as described above have already been industrially produced in large amounts and have been used in magnetic resonance imaging (MRI), office automation (OA) and factory automation (FA) equipment, various motors, actuators (VCM), a driving part of the printer head.
In the sintering process of Nd-Fe-B sintered magnet (hereinafter simply referred to as Nd-Fe-B magnet), the green compact powder is densified. An aim of the densification is as follows. In the well prepared powder, Nd-rich alloy powder, whose melting point is far lower than that of the Nd.sub.2 Fe.sub.14 B main phase, is uniformly dispersed, and the Nd-rich phase functions so that the liquid-phase sintering is realized. The liquid phase of Nd rich phase is distributed over the surface of the main-phase powder. The liquid-phase sintering enables densification at a relatively low temperature, without incurring grain growth appreciably.
Another important function of the Nd rich phase is to repair defects on the surface of the main-phase powder, which defects generate during the pulvering step. The most serious defects on the surface of main-phase powder are Nd-deficient layer formed due to preferential oxidation of Nd. The Nd rich phase supplies, from its liquid phase, Nd to this layer, thereby repairing the defects on the main-phase powder and hence enhancing the coercive force.
High densification of the sintered body is attained at a relatively low temperature by the liquid-phase sintering. However, it is desirable that the sintering temperature be high and close to the melting point of main phase and sintering be carried out for a long time.
However, when the sintering is carried out at high temperature and/or for a long time in the conventional methods, in a case that 3 .mu.m raw materials-powder is used, the crystal grains of main phase coarsen to 15 .mu.m or more, with the result that the coercive force of Nd-Fe-B magnet is lowered. The coercive force (iHc) of Nd-Fe-B magnet, which is obtained by an heretofore ordinary sintering method without coarsening the crystal grains of main phase, is approximately 12-13 kOe. The addition amount of borides is therefore limited to a relatively small amount.
The conventional Nd-Fe-B magnets are applied for OA and FA equipment, where environment is relatively moderate and of low-temperature and low-humidity.
It is known that the Nd-Fe-B magnets are less liable to rust in dry air than the SmCo magnets (R. Blank and E. Adler: The effect of surface oxidation on the demagnetization curve of sintered Nd-Fe-B permanent magnets, 9th International Workshop on Rare Earth Magnets and Their Applications, Bad Soden, FRG. 1987).
The Nd-Fe-B magnet is liable to rust in water or in a high humidity environment. As countermeasures for rusting liability of Nd-Fe-B magnet various surface-treatment methods, such as plating and resin-coating, are employed. However, since every coating by the surface treatment has defects, such as pinholes and cracks, water can intrude through the defects of coating to the surface of an Nd-Fe-B magnet and then vigorously oxidize the magnet. When the oxidation occurs, properties of a magnet are rapidly deteriorated and, rust, which floats on the surface of a magnet, impedes the functions of an appliance.
One of the previously proposed methods for improving the corrosion resistance to water, not relying on the surface treatment is that Al or Co is added to the Nd-Fe-B magnet. However, Al and Co can improve the corrosion resistance only slightly.
The corrosion resistance of Nd-Fe-B magnet is studied also from the view point of structure.
Sugimoto et al made a study on the mechanism of water-corrosion of Nd-Fe-B magnet (Corrosion mechanism of Nd-Fe-B magnet alloy. Sugimoto et al, Autumn Lecture Meeting of Japan Institute of Metals. No. 604, (October, 1987)). It has been clarified by this study that the corrosion speed in the water is in the following order of .circle.3 &gt; .circle.2 &gt; .circle.1 , wherein .circle.1 is Nd.sub.2 Fe.sub.14 B phase, .circle.2 is Nd rich-phase (e.g., Nd-10 wt % Fe), and .circle.3 is NdFe.sub.4 B.sub.4 phase (B rich phase), which phases constitute the sintered alloy having a standard composition of 33.3 wt % of Nd, 65.0 wt % of Fe, 1.4 wt % of B, and 0.3 wt % of Al.